This invention is related to a powder press for the production of pressed articles from powder-shaped pressing material, according to the patent claim.
It is known to produce moulded articles from hard metal, ceramics, sintered metal or the like with the aid of presses. The powder-shaped or granular metal, respectively, is formed to a formed body with the aid of presses, which is thereafter subjected to a sintering process. A press has an upper plunger and a bottom plunger, which co-operate with a female mould or a female die bore in a female mould plate. The plungers are driven by a suitable power drive, either mechanically or hydraulically via suitable hydraulic cylinders. It is known to arrange the female mould plate fixedly and to move the upper and the bottom plunger, or alternatively to move the female mould plate and to adjust the upper plunger, whereas the bottom plunger remains stationary.
It is essential for a reproducible production of powder pressed articles, that the pressed article receives a homogeneous structure at a predetermined density. Pressed articles with low density shrink more rapid upon sintering than pressed articles with higher density. It can be attempted to minimise density variations by pressing strokes which are differently adjustable for upper and bottom plungers. On the other hand, varying densities can be generated by varying pressing forces in practice, which themselves again are generated even at equal height of pressed articles, e.g. by reason of filling fluctuations, which may reach some percents. Hydraulic presses have the advantage that a relatively deliberately programmable pressing procedure can be achieved. From DE 4 209 767 C1 it is known to measure the pressing force in order to achieve a density which is uniform as much as possible in one batch. Depending on the measured pressing force, a correction is performed via the filling of the consecutive pressed articles thereafter. In this method, the position of the plungers is determined and the corresponding pressing force is determined. The starting-up from a particular position leads to a relatively high moulding precision, but causes varying densities in some cases . If on the other hand the pressing procedure is ended at a predetermined pressing force, there is the risk that the pressed article has varying thicknesses.
From DE 197 17 217 C2 it has become known to establish a desired force/path-diagram (set-point diagram) for one pressing plunger, in dependence of the geometry of the pressed article and the starting material during the compression in advance, and to memorise it. In a computer for the production process, the measured values for the path and the force of the pressing plunger are compared with the set-point curve during the compression. By means of a separately actuated portion of the press plunger or of a separate plunger, the pressure on the pressing material is increased or decreased during the compression phase, as soon as a deviation from the desired curve is detected. In this method, at least two position sensors have to be provided, namely for the at least one pressing plunger and the at least one further pressing plunger. From DE 100 10 671 C2 it has become known to establish a force/power-diagram for a desired pressed article through preceding experiments, according to which the bottom- or upper plunger is shifted during the compression. The follow-up according to the force/power-diagram requires also a control device, as is the case in the method described above.
From DE 1 042 772 C2 it has become known to memorise the value for the energy which is to be delivered by the upper plunger for a particular pressed article of predetermined geometry and dimensions and of predetermined material. Further, as a second value, the overall energy to be delivered by the upper and bottom plunger is memorised. During the compression, the forward feed of the upper plunger is ended when its energy consumption has reached the predetermined first memorised value, and the forward feed of the bottom plunger takes place according to the memorised remaining energy consumption and it is ended when the total energy has reached the second predetermined value. Even to do this, control is necessary.
It is the objective of all the methods, to ascertain with the aid of suitable controlling measures that always the same pressing result is obtained even with powder that varies within one batch.